Although Smokey the Bear signs in national forests and parks have warned the public for decades about the dangers of forest fires, the U.S. Forest Service regularly ignores the icon and sets controlled blazes as a means to prevent larger ones. Two reports in this week's issue of Science, however, argue that in some wildlands, it may be better to heed Smokey's hard-line stance.

In the 1950s, officials started using surplus military aircraft for fighting fires in forests in the western United States. By the 1970s, however, this strategy became a victim of its own success: Prevention was disrupting natural timber forest cycles, causing a buildup of inflammable debris that made later fires much more destructive. To reduce the woody debris, the Forest Service turned to controlled burns, a regime that was also adopted in brushlands and other ecosystems. Some have even advocated its use in tropical rainforests.

But Jon Keeley of the U.S. Geological Survey's Western Ecological Research Center in Three Rivers, California, who studies the dense, evergreen brushlands, or chaparral, in southern and central California, doubted whether the prevention philosophy had really led to larger, more destructive fires in the region. Keeley and his co-authors compared data on all recorded area fires from the late 19th century to the present (see paper[2]). They found no difference in the size or severity of fires before and after the introduction of aerial fire-fighting. And younger, thinner chaparral proved no more resistant than older areas to the catastrophic fires that perennially plague the region, casting doubt on the idea that controlled burns are an effective measure in chaparral.

In another study[3], Mark Cochrane, an ecologist with the Woods Hole Research Center in Woods Hole, Massachusetts, traced the effects of accidental forest fires in Brazil's western rainforest, where farmers practice slash-and-burn agriculture. Cochrane and his colleagues found that each previous fire made an area of forest more likely to burn again, with greater severity than the time before. The reason, Cochrane says, is that each fire adds dead wood to the forest floor and thins the forest canopy, allowing fuel wood to dry faster. Cochrane warns that unless current land use practices are changed, large swaths of Amazon rainforest could be irreversibly razed. "Maybe Smokey the Bear should move down here," he says.

Forest authorities had already begun to reconsider their fire-fighting policies, and the new insights should hasten that shift, according to Susan Conard, national program leader for fire ecology research at the U.S. Forest Service. She, too, thinks that large-scale prescribed burning of shrublands is probably not effective. But Conard says that setting fires in small, strategic areas is still an important tool for protecting towns and suburban areas.